Many articles of manufacture are made from metal castings with surfaces that are machined to a specified shape and to specified dimensions. Often such machining includes drilling or finishing of holes in the preformed body. Many automotive vehicle powertrain components are made of aluminum alloy castings that require hole drilling or hole finishing operations. Reaming is an example of a hole finishing operation. In this operation, a small amount of stock is removed from the inner diameter of the hole with a special tool called a reamer. Reamers are used in finishing operations for holes in cast aluminum alloy valve bodies for vehicle transmissions and in engine cylinder blocks and cylinder heads. The range of cutting operations is wide, requiring the use of many cutting tools and metal machining operations. The design of the cutting tools is important in maintaining machining operations without stopping for tool replacement or tool refurbishing.
Cutting tools with polycrystalline diamond inserts are used widely in machining of aluminum, its alloys, and other non-ferrous materials. In particular, round rotatable cutting tools with polycrystalline diamond (PCD) inserts are used in finishing operations to machine holes in aluminum alloy castings to final dimensions and roundness. The PCD insert is a body that provides a hard, durable cutting tip or edge for a machining operation. PCD-tipped tools are typically formed of a body of strong tool steel or metal carbide composition with one or more pairs of PCD cutting edges. The PCD cutting inserts comprise an inter-grown mass of randomly oriented, hard diamond particles sintered in a suitable matrix material. The cutting edges formed by the hard PCD material improve the effective life of the cutting tool and provide a better surface quality.
In addition to the PCD cutting edges, the tool body (for example, a tool intended to rotate in shaping a hole) may also have guide surfaces and/or edges that help position the tool with respect to the workpiece and reduce the tool round out. In particular, a PCD-tipped reamer may have surfaces and/or edges that follow the finished hole and help maintain concentricity of the tool and the hole, while the cutting surfaces remove aluminum alloy chips or particles. Sometimes these reamers have one or more pairs of radially extending guide surfaces and/or edges. In other cases, these reamers may have three radially extending guide surfaces and/or edges. The reamers are usually machined so that the guide surfaces and/or edges follow closely the machined hole but do not damage it and do not remove additional material. This is achieved by making the diameter of the guiding surfaces and/or edges a few microns smaller than the diameter of the PCD cutting edges. Guide surfaces and/or edges on a rapidly rotating tool engage and follow along the machined surface and reduce vibration of the cutting tool.
In addition to the guide surfaces and/or edges and the PCD cutting edges, the tool may also have chip evacuation surfaces to evacuate chips along the tool body and from the hole being cut. In reamers, these surfaces form flutes and sometimes have the shape of a circular spiral groove in the body of the tool. In other cases, these surfaces may be straight. In still other cases, these surfaces may have a combination of spiral and straight surfaces.
Thus, during the operation of the cutting tool, aluminum alloy chips are removed from the machined surface by the PCD cutting edges and removed from the cutting zone by sliding along chip evacuation surfaces. Sometimes, however, aluminum chips accumulate momentarily on the rotating guide surfaces and/or edges or chip evacuation surfaces. When chips adhere to and accumulate on the chip evacuation surfaces, the passages for the sequential chips are obstructed, resulting in material buildup, which scratches and/or mars the surface being machined. This accumulation of debris on tool surfaces is more rapid when the tool has been used for a period of time and has been worn. Left uncorrected, this accumulation of debris on tool surfaces can lead to decreased surface quality of the machined holes and/or to breakage of the tool. Thus, this condition may require untimely refurbishing of the cutting tool.
The cutting speeds are usually high during machining with PCD cutting tools. As a result, the removed chips are hot, which increases the probability that the chip will adhere momentarily to the non-cutting tool surfaces, such as guiding surfaces and/or edges or chip evacuation surfaces, resulting in chip accumulation and marring the surface. There is a need to minimize accumulation of aluminum material on guide surfaces and/or chip evacuation surfaces of PCD cutting tools.